Prior art voltage reference generators are exemplified by the circuit of FIG. 1 which was presented by Robert J. Widlar in an article entitled "New Developments in IC Voltage Regulators", IEEE Journal of Solid State Circuits, Volume SC-6, No. 1, pp. 2-7, February 1971. The value of R1 is 600 ohms, R2 is 6000 ohms and R3 is 600 ohms. V.sub.REF, the regulated output, is V.sub.BE +(R2/R3) V.sub.BE. Widlar stated,
". . . it uses the negative temperature coefficient of emitter-base voltage in conjunction with the positive temperature coefficient of emitter-base voltage differential of two transistors operating at different current densities to make a zero temperature coefficient reference. Practical references can be made at voltages as low as the extrapolated energy band-gap voltage level of the semiconductor material, which is 1.205 V for silicon. A simplified version of this reference is shown in Fig. [1, of this disclosure]. In this circuit, Q1 is operated at a relatively high current density. The current density of Q2 is about 10 times lower and the emitter-base voltage differential V.sub.BE between the two devices appears across R3. If the transistors have high current gains, the voltage across R2 will be proportional to V.sub.BE. Q3 is a gain stage that will regulate the output at a voltage equal to its emitter-base voltage plus the drop across R2."
The voltage across R2 was given as (R2/R3) V.sub.BE. Widlar, supra, p.3. This circuit develops a minimum output voltage which is close to the energy band gap voltage of silicon, 1.205 volts, and was stated to be temperature invariant at that voltage output level.
An article by A. Paul Brokaw, "A Simple Three-Terminal IC Bandgap Reference", IEEE Journal of Solid-State Circuits, Vol. SC-9. No. 6, December 1974, pp. 388-393 also teaches a circuit which is limited, at its lower outout level to the band-gap voltage of silicon, although Brokaw teaches a circuit which will produce regulated voltages which exceed the band-gap voltage. This referenced prior art depends upon on the equation: EQU V.sub.R =(V.sub.BE +K1.DELTA.V.sub.BE)K2 (a)
(If K1 and K2 in equation (a) are chosen to be equal to R2/R3 and 1.0, respectively: EQU V.sub.R =(V.sub.BE +R2/R3.DELTA.V.sub.BE)(b) (b)
is the result.)
Where K1 is a constant chosen so that: EQU dV.sub.BE /dT+K1(d.DELTA.V.sub.BE /dT)=0 (c)
and K2 is chosen to give the desired output voltage. It must be greater than 1.0 by definition since it is determined by a resistor divider (Brokaw) or is chosen to be 1.0 to insure proper circuit operation (Widlar), supra.
The unregulated source voltage for such circuits as taught by Widlar and Brokaw must have a minimum level of about 2.06 volts. In U.S. Pat. No. 4,100,477, Richard K. Tam teaches the regulator of FIG. 2 which also has the limitations expressed above. Among other things, Tam teaches the addition of resistor 18 to the basic Widlar circuit of FIG. 1. Other voltage regulator prior art which is known but is not deemed to be as relevant as the Widlar, Brokaw and Tam references is to be found in U.S. Pat. Nos. 2,617,859 to Dobkin et al.; 3,659,121 to Fredericksen; 3,781,648 to Owens; 3,794,861 to Bernacchi; 3,886,435 to Steckler; 3,970,876 to Allen et al.; 3,893,018 to Marley; 4,091,321 to Hanna; 4,339,707 to Gorecki; 4,362,984 to Holland; and 4,447,784 to Dobkin.